Adds _Record.affected_start and .affected_end.

These coordinates should represent the zero-based, half-open region of the reference sequence affected by all the events included in ALT. These coordinates allow the user to identify precisely which bases are altered by the events in the record.

Provides more thorough documentation on the coordinate schemes for _Record.POS, .start, and .end.

This PR is a re-submission of an original attempt to capture the coordinates of the affected region, PR #160.